Ethylene oxide (EO) is an intermediate product for making a wide variety of derivatives, the most important derivatives in terms of volume being the ethylene glycols (EG). Other derivatives include ethanolamines, glycol ethers, detergent ethoxylates, polyols and others. EG is produced by the reaction of EO with water, producing mono ethylene glycol (MEG) as the main product with di ethylene glycol (DEG) and tri ethylene glycol (TEG) as by-products.
MEG is mainly used for the manufacture of polyester fibres, polyethylene terephthalate (PET) and, to a lesser extent, in the cooling systems of motor vehicles where it serves as antifreeze. DEG is also used in the fibre industry and as tobacco humectant. DEG and TEG are both used for gas drying. TEG is also used in the manufacture of cellophane for food packaging.
The production of ethylene oxide is described in Kirk-Othmer Encyclopedia of Chemical Technology, 3rd edition, Volume 9, 1980, pages 443 to 447. The production of ethylene glycols is described in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, Volume A 10, 1987, pages 104 and 105.
EO is often produced in a combined EO/EG process, which has the advantages that the EO process intrinsically has some glycols formation that requires work up, and that the combined process provides very efficient heat integration. The integrated process is usually split up into four sections: EO reaction and recovery; Light ends (LE) removal and EO purification; Glycols reaction and dewatering; and Glycols purification.
The present invention has application in the first section, EO reaction and recovery, which comprises an EO reactor in which EO is produced by catalytic gas phase oxidation of ethylene with a molecular oxygen-containing gas, a recycle loop for recycling unconverted reagents and removing products (EO, water and CO2) and supplying fresh feeds, an EO Absorber and Stripper for EO recovery from absorbent by absorption in water and concentration, and a CO2 removal section.
The general process conditions that apply for the EO recovery section in the prior art and also for the present invention are suitably an elevated temperature pressure. Herein an elevated temperature indicates a temperature above ambient. Similarly elevated pressure indicates a pressure in excess of atmospheric pressure. The recovery section may however operate at a temperature in the range of from ambient, for example 20° C., to 150° C., and a pressure in the range of from 100 to 1,000 kPa, for example from 200 to 400 kPa. An EO Stripper column will operate with a temperature and pressure differential from top to bottom.
In a prior art system shown in FIG. 1, loopgas (not shown) from an EO Reactor enters the bottom part of the EO Absorber (1). The gas travels upwards and is washed with Lean Absorbent (LA) which is substantially EO-free. The almost EO-free top gas (not shown) is recycled back to the EO reactor system. In the EO absorber the EO is absorbed in the LA and exits as Fat Absorbent (FA) (11). FA (12) from the Residual Gas Absorber (RA) (2) is added to this stream. Cold FA (11, 12) flows through LA/FA heat exchanger (3) and is heated up.
The heated FA (13) enters the top of the EO Stripper (4). An EO/water mix leaves the EO Stripper over the top (14). In the bottom part of the EO Stripper heat is supplied by live steam or by means of a reboiler. Here also some LA is removed as glycol bleed and glycol removed from the LA. The water balance of the system is adjusted with a live steam or condensate stream (15) to the EO Stripper (4). Hot LA leaves the EO Stripper (4) over the bottom (16). In the LA/FA heat exchangers (3) the hot LA is cooled down. To get the correct temperature for absorption the LA is further cooled in other heat exchangers (5). The LA is then available for re-use in the EO Absorber (1) via line 17 and the RA (2) via line 18.
Analysis of the recovery section in one plant showed that in the EO Stripper approximately 30 MW of heat is used, whereas for the evaporation of EO only 7 MW is required. The explanation that much more heat is required in that plant is that about 17.5 MW is used to heat up LA and 7 MW is used to evaporate water out of the top of the EO Stripper. Moreover some impurities are present in the EO Stripper overhead which negatively affect the downstream EG process.
Accordingly there is a need to conserve energy which is currently consumed in evaporating a significant amount of water and removing it as vapour, and which appears to be unnecessary for separation purposes. Moreover there is a need to remove impurities originating in the EO process.
U.S. Pat. No. 4,875,909 (NSKK) discloses a method for the recovery of EO from such a process in order to reduce the heating energy of the EO Stripper. In this publication a prior art line up is disclosed in which a reduction of water in the EO Stripper overhead is achieved by creating a rectification in the EO Stripper with reflux of (partly) condensed top product. This refluxed material has to be evaporated again, costing energy. In this line-up, hot FA is fed to the upper part of an EO Stripper and (partly) condensed EO Stripper overhead vapor is returned to the top of the EO Stripper.
U.S. Pat. No. 4,875,909 discloses further line-ups to recover more heat from the top vapour of the EO Stripper and to recover heat from the EO Stripper bottoms using the following methods:
flash the bottom stream and apply vapour recompression of the flashed steam and use that steam in the EO Stripper
use the bottom stream further as a heat medium in a refrigerator system
use the bottom stream (after being used in other heat recovery possibilities) as a heat source in the EO Refiner located in the EO Stripper bottom stream, or in the LE Column.
In this publication the third characteristic of the apparatus and method relates to the heat content of the diffusate from the top of the EO Stripper. Lower heat content is achieved by lower water concentrations of the EO Stripper tops. This again is achieved by a reflux with partly condensed overhead vapors of the EO Stripper (in a reboiler of the LE Column). With less water going over the top, also less water has to be evaporated.
To make use of the heat content of the EO Stripper tops of this publication, even under conditions where it contains much less water, the pressure can be increased, in that way increasing the dew point. The heat content can then be used as a heat source for the EO Refiner.